Not only slit lamps used solely for diagnostic purposes, but also so-called laser slit lamps are known in ophthalmology. The optical configuration of these laser slit lamps is always such that it allows the therapeutic use of suitable laser wavelengths in the patient's eye. In addition to other optical components, standard laser slit lamps feature a safety filter in the viewing beam path of the slit lamp microscope in order to absorb as much of the laser radiation as possible. This prevents reflected laser radiation from reaching the eye of the physician and putting him at risk.
Conventional diagnostic slit lamps which do not offer any possibility of laser treatment normally do not feature such a safety filter.
Various manufacturers are now providing retrofit kits which allow the conversion of conventional diagnostic slit lamps into laser slit lamps using a so-called laser adapter. When a laser adapter is retrofitted to a conventional diagnostic laser slit lamp, a suitable safety filter for the physician must also be provided for safety reasons.
Various possibilities exist for mounting such a safety filter in the laser adapter. It is possible, for example, to mount this safety filter as a stationary unit at a defined point in the viewing beam path of the slit lamp. This has the disadvantage that the physician always looks through this filter at the treatment site irrespective of whether the laser has been activated or not. This safety filter has an absorbing effect usually in a wavelength range between 460 to 540 nm. For this reason, a pronounced color distortion in the visible spectral range results, however, with such a safety filter. The above wavelength range is always filtered out of the visible spectral range, resulting in an orange-colored image for the physician. This color distortion is perceived to be irritating.
A known alternative uses a motor to pivot the safety filter into the beam path only during the actual laser treatment, that is, when the laser is active. Thus, the color distortion described above is only present during the actual laser treatment. If such a pivotable safety filter is to be provided in a laser adapter retrofitted to conventional diagnostic slit lamps, the only possibility is to swing in the safety filter between the objective lens or the two objective lenses of the slit lamp microscope and the observed target plane when the laser has been activated.
U.S. Pat. No. 4,520,816, for example, illustrates the prior art pertinent for this type of arrangement and is incorporated herein by reference. If, in the arrangement disclosed in this patent, the safety filter is swung into this position in the viewing beam path of the slit lamp microscope when the laser is activated, the focal intercept of the optical system of the slit lamp microscope will be changed for as long as the filter remains in the beam path. During this period, the physician will see a blurred image, and a degree of uncertainty may result for focusing the laser beam on the target plane which must be done with high precision.